1. Field of the Invention
The present invention relates generally to a method for manufacturing inductor devices, and more particularly to a fabrication method of high performance integrated inductor devices using a substrate conversion technique.
2. Description of the Related Art
Recently emerging need for radio frequency (RF) integrated circuits (IC) for such markets as cellular telephones, wireless modems, and other types of communication equipment, has pushed silicon technology to several gigahertz applications. Therefore, attempts to integrate active elements and passive elements such as inductor into one chip have prompted interest in monolithic RF IC. In the RF IC, the inductor is used for an impedance matching.
The major factor for high performance integrated inductor is an quality factor (Q). The objective of high-Q inductor designs is to increase inductance and decrease resistance, while keeping parasitic capacitance to a minimum so that high oscillation frequencies can be achieved. However, lossy characteristic of Si substrate has prohibited the fabrication of high Q inductor and low loss transmission lines on the Si technology.
FIG. 1 illustrates a conventional spiral inductor structure in (A) cross-sectional view and (B) plan view. Referring to FIGS. 1(A) and 1(B), a first interlayer dielectric 2 with active elements such as CMOS (not shown) is formed on a silicon substrate 1. Then a first metal level 3, a second interlayer dielectric 4, a second metal level 6 and a passivation layer 7 are sequentially formed on the first interlayer dielectric 2. The first metal level 3 is covered by the second dielectric 4 through which a via hole 5 is formed. The first metal wire 3 is used as a cross-under to make connection to the second metal wire 6 via a contact hole 5. The spiral coil is built at the second metal level 6.
In this spiral inductor structure, the parasitic capacitance becomes heavily dependant to the thickness of the second insulator 4 used as a dielectric material. That is, the decreasement of the parasitic capacitance is very difficult because of the limitation of the dielectric thickness. As a result, the conventional art has the difficulty in producing a high performance inductor in silicon that is suitable for RF applications.
So, till now, GaAs substrate is mainly used for monolithic RF in virtue of its semi-insulating characteristic and high mobility. Modern Si IC process offers good active elements, but it does not provide passive elements like inductor with acceptable performance. This is due to the semiconducting nature of the silicon substrate. There have been several efforts to fabricate the inductor on the Si substrate, but the obtained performance such as Q factor, resonant frequency are not good enough.